1. Field of the Invention
The present invention relates to a diaphragm used for a speaker, and particularly relates to a balance dome-type diaphragm including a cone portion at the periphery of a dome portion, a speaker including the diaphragm, and the method for manufacturing the speaker diaphragm.
2. Description of the Related Art
With widespread use of high-resolution audio, a speaker being able to reduce disturbance of sound pressure frequency properties of a high-tone range including an extremely high-tone range of equal to or higher than 20 kHz has been recently developed. Generally, a metal-based diaphragm exhibits a higher stiffness and a higher high-tone range threshold frequency as compared to a resin-based diaphragm, and therefore, is suitable for high-tone range reproduction. Of diaphragm materials, magnesium or magnesium alloy is the most suitable metal material for high-tone range reproduction because such a material has a lower specific gravity and exhibits less sound pressure reduction as compared to aluminum and titanium.
For example, Japanese Patent No. 4152804 describes a dome-type diaphragm configured such that a dome portion and an edge are integrally formed of a thin magnesium sheet, the magnesium sheet being formed in such a manner that a magnesium base material is rolled several times with different rolling amounts.
However, as the crystal structure of magnesium is a hexagonal close-packed structure, magnesium is strongly plastically anisotropic and is less likely to stretch. For these reasons, it is difficult to perform plastic working for magnesium or magnesium alloy. Thus, such a material can be processed into a simple shape as in the dome-type diaphragm described in Japanese Patent No. 4152804, but it is difficult to bend a magnesium sheet into a complicated shape such as a balance dome shape including a cone portion at the periphery of a dome portion and being suitable for output with a high-tone range. For this reason, the dome portion and the cone portion are, in a typical case, separately formed from a magnesium or magnesium alloy sheet material, and are bonded together with an adhesive. In this manner, a balance dome-type diaphragm is formed. In this case, a joint line is formed along a boundary between the dome portion and the cone portion of the diaphragm. This leads to a problem that a sound pressure level is lowered due to an adhesive weight load, and therefore, sound pressure frequency properties are disturbed. Moreover, the dome portion and the cone portion of the speaker diaphragm are bonded with the adhesive, leading to a problem that a manufacturing cost increases due to an increase in the number of working processes.